1. Field of the Invention
This invention relates to a stencil printer, and more particularly to a stencil printer which prints by the use of a stencil wound around a printing drum.
2. Description of the Related Art
There has been known a stencil printer in which printing papers are inserted into between a rotating printing drum around which a stencil is wound and a press roller which is rotated in contact with the printing drum under a pressure, and ink supplied inside the printing drum is transferred to the printing papers through the perforations in the stencil.
The printing papers are generally fed to the printing drum by primary and secondary paper feed sections which are driven by the printing drum by way of a driving mechanism employing gears and the like. The primary and secondary paper feed sections are controlled so that each of the printing papers is accurately positioned with respect to the stencil on the rotating printing drum.
In the primary paper feed section, printing papers on a paper feed table are fed out one by one, one each time the printing drum makes a rotation, by a pickup roller and a scraper roller. The pickup roller has a friction member coaxially fixed to a shaft which is intermittently rotated in response to engagement and disengagement of a paper feed clutch. Then the printing papers are transferred to the secondary paper feed section. The pickup roller and the scraper roller are provided with a one-way clutch and when the paper feed clutch is disengaged after the printing paper is delivered to the secondary paper feed section, the pickup roller and the scraper roller are rotated driven by way of the printing paper, thereby reducing the back tension.
In the secondary paper feed section, the leading end of the printing paper abuts against the contact line of a guide roller and a timing roller (the two rollers will be referred to as xe2x80x9cthe paper feed roller pairxe2x80x9d, hereinbelow) which are stopped or a surface of the paper feed roller pair near the contact line. After the printing paper is thus provided with sag, the paper feed roller pair are started at a predetermined angular position of the printing drum. The paper feed roller pair are in mesh with each other at their ends and the guide roller is drivingly connected to a main motor by way of a driving force transmitting means such as gears, an endless belt and the like. The guide roller is arranged to make a predetermined number of rotations per one rotation of the printing drum by a mechanism including a cam, a sector gear, a one-way clutch and the like. The timing roller is driven by the guide roller in the reverse direction. The timing roller is arranged to be moved away from the guide roller after the printing paper is delivered to the printing drum and the guide roller is stopped by a mechanism including a cam, a cam follower, a link, a resilient member and the like. A spring, an electromagnetic brake and the like are provided on one end portion of the timing roller in order to suppress the delay between disengagement of the timing roller from the guide roller and stop of the guide roller due to inertia.
The printing paper delivered to the printing drum by the paper feed roller pair is pressed against the stencil on the printing drum under a predetermined pressure by the press roller and ink supplied by an ink supply section disposed inside the printing drum is transferred to the printing paper through the perforations in the stencil, whereby a print is made.
Conventionally, the paper feed roller pair are arranged to be opened, that is, to be moved away from each other, just at the time, the leading end portion of the printing paper comes to be pinched by the printing drum and the press roller (i.e., the printing papers comes to be driven by the printing drum and the press roller) which are rotating. This involves the following problems.
When the paper feed roller pair are prematurely opened just before the leading end portion of the printing paper comes to be pinched by the printing drum and the press roller while a trailing end portion of the printing paper is still in the primary paper feed section, the primary paper feed section, that is, the pickup roller, the scraper roller and the like, applies back tension to the printing paper, which can result in stretch of the stencil (which causes stretch of printed image generally called xe2x80x9cghostxe2x80x9d) or displacement of the stencil. To the contrast, when the paper feed roller pair are opened after the leading end portion of the printing paper comes to be pinched by the printing drum and the press roller, the paper feed roller pair apply back tension to the printing paper, which can also result in ghost and/or displacement of the stencil.
Accordingly, it has been necessary to highly accurately control the timing of opening the paper feed roller pair, which adds to the cost of the stencil printer.
Further, when the paper feed roller pair are opened and closed, noise is generated.
Further, printing papers vary widely in thickness and the paper conveying rate of the paper feed roller pair for a given speed of the paper feed roller pair varies according to the thickness of the printing paper, which makes it necessary to change the timing of opening the paper feed roller pair according to the thickness of the printing paper. Further, depending on the material of the paper feed roller pair, the diameter of the paper feed roller pair changes with the temperature of the environment. When the diameter of the paper feed roller pair changes, the paper conveying rate of the paper feed roller pair for a given speed of the paper feed roller pair varies, which makes it necessary to change the timing of opening the paper feed roller pair according to the temperature of the environment.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil printer in which generation of ghost can be suppressed without accurately controlling the timing of opening the paper feed roller pair, whereby the cost can be reduced.
Another object of the present invention is to provide a stencil printer in which the printing papers can be stably conveyed irrespective of the thickness of the printing papers or the temperature of the environment without necessity of accurately controlling the timing of opening the paper feed roller pair.
Still another object of the present invention is to provide a stencil printer which can prevent generation of noise due to opening and closing of the paper feed roller pair.
The stencil printer in accordance with the present invention comprises
a printing drum which is rotated bearing a stencil wound around the printing drum,
a pair of paper feed rollers which are rotated in contact with each other to feed a printing paper inserted therebetween and supply it to the printing drum,
a pressing roller means which is rotated in contact with the printing drum to convey the printing paper supplied to the printing drum with the printing paper pressed against the stencil on the printing drum,
a paper feed roller drive means which drives the paper feed rollers, and
a paper feed roller control means which controls the paper feed roller drive means to rotate the paper feed rollers at a peripheral speed higher than that of the printing drum at least from the time at which the leading end of the printing paper reaches the printing drum to the time at which the trailing end of the printing paper passes the paper feed rollers.
It is preferred that the paper feed roller control means be provided with a speed changing means which controls the paper feed roller drive means to change the peripheral speed of the paper feed rollers.
For example, the speed changing means may control the paper feed roller drive means to change the peripheral speed of the paper feed rollers according to the paper conveying rate of the paper feed rollers.
The speed changing means may be provided with a sag detecting means which detects the amount of sag of the printing paper between the paper feed rollers and the printing drum and may control the paper feed roller drive means to change the peripheral speed of the paper feed rollers according to the amount of sag.
In the stencil printer of the present invention, since the paper feed rollers are rotated at a peripheral speed higher than that of the printing drum at least from the time at which the leading end of the printing paper reaches the printing drum to the time at which the trailing end of the printing paper passes the paper feed rollers, the printing paper is conveyed with sag formed between the paper feed roller pair and the printing drum. Accordingly, back tension applied to a trailing end portion of the printing paper by the primary paper feed section or the secondary paper feed section is hardly transmitted to the leading end portion of the printing paper in contact with the printing drum (or the stencil), whereby generation of ghost or displacement of the stencil can be prevented.
Further, since the printing paper is delivered to the printing drum with the printing paper pinched by the paper feed roller pair, the paper feed roller pair need not be opened and accordingly, the mechanism for accurately controlling the timing of opening the paper feed roller pair may be eliminated, whereby the cost can be reduced.
When the paper feed roller pair are not opened and closed, generation of noise due to opening and closing of the roller pair can be prevented.
When the paper feed roller control means is provided with a speed changing means which controls the paper feed roller drive means to change the peripheral speed of the paper feed rollers, the printing papers can be conveyed at an optimal speed according to, for instance, the kind of the printing papers and the condition of the environment of the stencil printer, whereby the printing papers can be constantly conveyed stably irrespective of the kind of the printing papers, the condition of the environment of the stencil printer and the like.
When the sag in the printing paper becomes too large, the printing paper can be brought into contact with the printing drum and stained with ink before printing. This problem can be avoided by controlling the peripheral speed of the paper feed rollers so that the sag in the printing paper does not grow too large.